Realtime pipeline schedule statuses

What does this MR do and why?

Part 1: !200990 (merged)

Part 2: YOU ARE HERE 👋

This MR adds realtime pipeline schedules, so users get the latest info on pipeline schedules without polling or refreshing the page.

References

Screenshots or screen recordings

Demo in action 📹

https://youtu.be/2YbK1sZs1L8?feature=shared

Screenshot

How to set up and validate locally

1. Enable FF :ci_pipeline_schedules_status_realtime in rails console
2. Have CI setup with a pipeline schedule
3. Visit schedule and trigger it
4. Watch the status update in real-time

Helpful way to test in rails console

pipeline = Ci::Pipeline.find(YOUR_ID)

# Test multiple status changes

pipeline.run!
# Check subscription fired with "running"

pipeline.succeed!
# Check subscription fired with "success"

pipeline.run!
# Check subscription fired again with "running"

pipeline.drop!
# Check subscription fired with "failed"

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #561503 (closed)